100% j15 motor 70-75HP
dont bother with the a series the 620 is a heavy car
pull the angle grinder out and take those engine mounts off at the weld
then find a car with an l20b and pull the engine and gearbox out of it

measure the tip of the gearbox of the j15 to the mounts on the j15

then measure the same on the l20b, you want this measurement to match up as close as possible to the j15 measurement
if you cant find one close enough you will have to get your tailshaft shortened